Method for producing a hollow structural part made of fiber-reinforced plastic

ABSTRACT

For production of a hollow structural part made of fiber-reinforced plastic, a water-dispersible support core made of a water-soluble binding agent consisting at least partly of a water-soluble silicate-containing binding agent, and a filler is wrapped with the reinforcing fibers. The fibers on the support core are impregnated with a curable plastic, the plastic is cured, and the support core is subsequently flushed out with water.

This invention relates to a method for producing a hollow structural part made of fiber-reinforced plastic according to the preamble of claim 1.

For production of hollow fiber-reinforced plastic parts there are employed inter alia the so-called RTM (resin transfer molding) process and the vacuum injection process.

For this purpose, at least one layer of the e.g. uni- or bidirectionally oriented reinforcing fibers is placed between the upper and lower tools of a heated press and the fiber layer impregnated with a heat-curable plastic, for example an epoxy resin with curing agent, which is injected under pressure into the cavity with the fiber layer between upper and lower tools. The procedure is similar in the vacuum injection method whereby, instead of pressure, a vacuum is applied to suck the heat-curable plastic into the fiber layer.

For automobiles there are employed numerous hollow structural parts, for example the support pillars, door sills, bumpers and the like. Such hollow parts are usually bonded. However, the bonding point can lead to failure. In addition, the dimensional accuracy of bonded hollow parts leaves much to be desired.

For production of hollow parts by the RTM process it is customary today to use melt cores made of wax. The processes used therefor are very elaborate because of the size of the cores. Further, the great thermal expansion coefficient of the wax requires an elaborate adjustment of the necessary manufacturing facilities. After the actual part production the materials are melted out again. This leaves residual material on the inner tool wall, which firstly increases the part weight and is also judged critically with regard to emissions and lacquer compatibility.

From WO 02/072328 A1 there is known a support core for production of hollow structural parts made of plastic which contains polyvinylpyrrolinone (PVP) as the water-soluble binding agent.

In order to wrap the support core there can be employed a braiding machine, whereby the support core is located in the eye of the braiding machine while the reinforcing threads are pulled off from the periphery under high tension. However, the support core according to WO 02/072328 A1 possesses too little breaking strength to withstand such high pull-off forces.

There have also been employed support cores made of low-melting bismuth alloys. However, because of the high energy input for melting the cores, the high weight and the resulting difficult handling, but also because of the health risk of bismuth vapors, such cores are not employable in practice.

Further, use is made of support cores made of high-density foamed material which remain within the part and thus lead to a corresponding weight increase.

The object of the invention is hence to provide, for an RTM process for production of hollow structural parts made of fiber-reinforced plastic, a water-dispersible support core that safely withstands the high pull-off forces upon wrapping with the reinforcing fibers.

This is achieved according to the invention by the method characterized in claim 1. In the subclaims there are stated advantageous embodiments of the invention.

The support core employed according to the invention is characterized by the fact that the water-soluble binding agent contains at least partly a silicate. The binding agent preferably employed is a water glass. The binding agent is characterized in that the SiO₂/M₂O weight ratio is preferably in the range of from 1.6 to 4.0, in particular from 1.8 to 3.5, where M signifies sodium ions and/or potassium ions and/or lithium ions. The binding agent preferably has a solids content of SiO₂ and M₂O in the range of from 30 to 60 wt %.

The support core has a relatively small content of binding agent of preferably from 0.5 to 8 wt %, in particular from 1 to 5 wt %, and thus an accordingly high filler content. This achieves high strength, so that it withstands the high pull-off forces upon packing, i.e. wrapping, in particular braiding, with the reinforcing fibers.

The reinforcing fibers used are preferably carbon fibers. However, it is also possible to employ other reinforcing fibers, for example glass fibers. The packing is preferably carried out with a braiding machine as employed for braiding cables and ropes, whereby the support core occupies the place of the cable core or rope core in the eye of the machine.

The production of the fiber-reinforced hollow structural part is then preferably effected by the RTM process. That is, the packed support core is placed between the two tools, for example, the upper and lower tools of a heated press, whereupon the reinforcing fibers on the support core are impregnated with a heat-curable plastic, for example an epoxy resin which is injected into the cavity with the support core between the two tool parts. It is possible here to carry out the resin transfer molding (RTM) process by which the heat-curable plastic is pressed into the cavity under a pressure of 10 bar and more, or the vacuum injection process by which the heat-curable plastic is sucked into the cavity. After curing of the resin, demolding is effected and the support core flushed out with water, so that the fiber-reinforced hollow structural part is formed.

The employed water glass can be for example a sodium silicate. However, it is also possible to employ other water glasses, in particular potassium silicate or also lithium silicate.

Besides water glass, the binding agent can contain further alkali metal compounds which can be added in solid form or as an aqueous solution, for example sodium hydroxide or potassium hydroxide or, in dissolved form, caustic sodium or caustic potash. Said additives can be used for example to control the modulus (SiO₂/M₂O ratio) and the solids content.

The filler consists of a water-insoluble particulate material, preferably at least partly of sand. The sand preferably has an average grain size of from 100 to 500 μm.

Besides the strength that it gives to the support core, the sand also has an important function in the production of the support core. The support cores for the inventive method are preferably produced by a core shooting machine as described for example in DE 102 00 927 A1 for the production of cores for foundry purposes. Here, a mixture of foundry sand, water-dissolved magnesium sulfate as a binding agent and water is shot into a molding tool. The water is subsequently evaporated to form the foundry core for production of the casting.

In order for the mixture of filler, water-dissolved binding agent and water, which is preferably employed for shooting according to the invention, to have the fluidity necessary for shooting into the molding tool, it has proved expedient that the sand content of the filler is at least 30 wt %.

Further, the filler preferably contains plastic particles, normally with an average grain size of from 100 μm to 2 mm. The plastic particles are preferably formed from shredded plastic waste. In order for the particles to be of rather spherical configuration, it is preferable to employ plastic pellets. The spherical shape accordingly increases the above-mentioned fluidity of the mixture of filler, dissolved binding agent and water. The plastic particles in addition increase the elasticity of the support core, i.e. reduce its brittleness. At the same time, the weight of the support cores can be substantially lowered by the use of plastic particles as the filler. Preferably, the content of plastic in the filler is at least 10 wt %. However, it can also be considerably higher, amounting to 50 wt % and more. This makes it possible to obtain substantially lighter and thus accordingly more easily handled cores.

A weight reduction of the cores is further attainable with ceramic particles or hollow glass spheres. The ceramic particles can be formed for example by fly ash. Because the fly ash particles are likewise of rather spherical configuration, they at the same time ensure the fluidity necessary for shooting. This is of course especially true of hollow glass spheres as a filler. The average particle size of the ceramic particles and the hollow glass spheres can be for example from 50 to 800 μm.

The water content of the molding material, i.e. of the mixture of filler, binding agent and water, is preferably from 0.2 to 5 wt % based on the weight of the filler. This ensures the processability of the molding material upon insertion into the molding tool, for example, by shooting or pressing. However, the water content is chosen as low as possible, because the support core must be dried before demolding in order for it to possess the necessary strength.

A preferred molding material consists for example of from 0.5 to 5 wt %, in particular from 1 to 2 wt %, of silicate-containing water-soluble binding agent, with a SiO₂/M₂O ratio preferably in the range of from 1.6 to 4.0, where M signifies sodium ions and/or potassium ions and/or lithium ions, and a solids content of from 30 to 60%; and balance sand.

According to the inventive method there can be produced any hollow structural parts made of fiber-reinforced plastic. In the automobile sector there are to be mentioned in particular the support pillars, i.e. the A-, B- and C-pillars, door sills and bumpers as examples of such fiber-reinforced hollow structural parts. 

1. A method for producing a hollow structural part made of fiber-reinforced plastic wherein a water-dispersible support core made of water-soluble binding agent and filler is wrapped with the reinforcing fibers, whereupon the fibers on the support core are impregnated with a curable plastic, the plastic is cured, and the support core is flushed out with water, characterized in that the binding agent consists at least partly of silicatic components.
 2. The method according to claim 1, characterized in that the binding agent consists at least partly of water glass.
 3. The method according to claim 1, characterized in that the content of binding agent is from 0.5 to 8 wt %, preferably from 0.8 to 4 wt % and particularly preferably from 1 to 2.5 wt %, based on the weight of the filler.
 4. The method according to claim 2, characterized in that the binding agent has a SiO₂/M₂O weight ratio of from 1.6 to 4.0, in particular from 1.8 to 3.5, where M signifies sodium ions and/or potassium ions and/or lithium ions.
 5. The method according to claim 1, characterized in that the binding agent has a solids content of SiO₂ and M₂O in the range of from 30 to 60 wt %.
 6. The method according to claim 1, characterized in that the binding agent consists at least partly of sodium silicate and/or potassium silicate and/or lithium silicate.
 7. The method according to claim 1, characterized in that the binding agent contains, besides the water glass, at least one further alkali metal compound.
 8. The method according to claim 7, characterized in that the further alkali metal compound is a sodium compound and/or potassium compound.
 9. The method according to claim 1, characterized in that the filler consists at least partly of sand.
 10. The method according to claim 1, characterized in that the filler consists at least partly of plastic particles.
 11. The method according to claim 1, characterized in that the filler consists at least partly of ceramic particles.
 12. The method according to claim 10, characterized in that the plastic particles or ceramic particles are of spherical configuration.
 13. The method according to claim 1, characterized in that the filler consists at least partly of ceramic or hollow glass spheres.
 14. The method according to claim 1, characterized in that it is carried out as a resin transfer molding process.
 15. The method according to claim 1, characterized in that it is carried out as a vacuum injection process. 